Design Challenges and Solutions for the I-205 Abernethy Bridge Retrofit and Widening Project-Paper

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The I-205 Abernethy Bridge in Oregon is undergoing seismic retrofit and widening to improve its performance as a priority regional emergency-response lifeline. The project adds lanes and upgrades the approximately 2,700-foot bridge, which crosses highly variable ground conditions above the Willamette River.<br /><br />Design had to meet two seismic performance levels: remain operational after a deterministic full-rupture Cascadia Subduction Zone earthquake, and avoid collapse under probabilistic 1,000-year ground motions. Extensive field and laboratory investigations informed nonlinear site-response analyses that assessed soil strength, liquefaction, pore-pressure generation, and permanent ground deformation. Analyses showed that deep lateral movements—locally extending to about 150 feet—could impose severe demands on existing foundations.<br /><br />The design team combined ground improvement with new and upgraded foundations. Jet grouting and deep soil mixing were used at selected riverbank locations to reduce critical subsurface deformation, with layouts shaped by access, existing structures, and environmental and right-of-way constraints. Because improvement could not eliminate all movement, many river piers received replacement “outrigger” bents, connected to the bridge through isolation bearings. These are supported by large-diameter drilled shafts, generally 8 to 12 feet wide; some shafts exceed 240 feet in length and are socketed into basalt. Permanent casing was added to certain shafts to increase flexural resistance. On the West Linn hillside, where ground movements were smaller, existing foundations were strengthened with micropiles and footing anchors, while new shafts support the widened bridge.<br /><br />The project demonstrates how integrated geotechnical and structural modeling can guide seismic design for variable ground conditions and large permanent soil displacements. Its unusual shaft dimensions and depths also raised constructability questions, prompting a multi-shaft bidirectional load-testing program. Construction began in 2022, with completion expected before the end of 2025.

Keywords

I-205 Abernethy Bridge

seismic retrofit

bridge widening

Willamette River

Cascadia Subduction Zone

liquefaction

jet grouting

deep soil mixing

drilled shafts

micropiles